Several controlled pressure drilling techniques are used to drill wellbores with a closed-loop drilling system. In general, controlled pressure drilling includes managed pressure drilling (MPD), underbalanced drilling (UBD), and air drilling (AD) operations.
In the Managed Pressure Drilling (MPD) technique, the drilling system uses a closed and pressurizable mud-return system, a rotating control device (RCD), and a choke manifold to control the wellbore pressure during drilling. The various MPD techniques used in the industry allow operators to drill successfully in conditions where conventional technology simply will not work by allowing operators to manage the pressure in a controlled fashion during drilling.
During drilling, for example, the bit drills through a formation, and pores become exposed and opened. As a result, formation fluids (i.e., gas) can mix with the drilling mud. The drilling system then pumps this gas, drilling mud, and the formation cuttings back to the surface. As the gas rises up the borehole in an open system, the gas expands and hydrostatic pressure decreases, meaning more gas from the formation may be able to enter the wellbore. If the hydrostatic pressure is less than the formation pressure, then even more gas can enter the wellbore.
A core function of managed pressure drilling attempts to control kicks or influxes of fluids as described above. This can be achieved using an automated choke response in a closed and pressurized circulating system made possible by the rotating control device. A control system controls the chokes with an automated response by monitoring flow in and out of the well, and software algorithms in the control system seek to maintain a mass flow balance. If a deviation from mass balance is identified, the control system initiates an automated choke response that changes the well's annular pressure profile and thereby changes the wellbore's equivalent mud weight. This automated capability of the control system allows the system to perform dynamic well control or CBHP techniques.
The chokes of the manifold have a non-linear response. This can make it difficult to determine the true position of the chokes and properly control pressure and flow as conditions change. Techniques are available in the prior art to calculate the true position of the chokes so that a desired flow rate or pressure drop can be produced. To do the calculations, these prior art techniques may use variables, such as flow coefficient, characteristic of the valve, discharge coefficient, etc.
For instance, U.S. Pat. No. 8,352,087 discloses a way to control a flow control valve by determining a corresponding flow coefficient Cv from a curve of the valve using a measured valve opening, calculating a pressure drop from a measured flow and the valve's determined flow coefficient, calculating a flow error between the set point flow and measured flow, calculating a corrected flow value by adding the set point flow to the integrated flow error, calculating a new flow coefficient Cv from the corrected flow and the calculated pressure drop, and finally determining a new valve opening corresponding to the new flow coefficient. From this, the position for the new valve opening to meet the desired flow is used to control the valve so that the technique can attempt to linearize the relationship between control and flow.
U.S. Pat. No. 7,636,614 discloses a non-linear dynamic model for a control valve to account for physical changes to a valve's dead band and flow coefficient due to mechanical wear of the valve during operation. The current dead band and flow coefficient determined for the valve can be used to update valve control algorithms and to make maintenance decisions for the valve.
U.S. Pat. No. 7,769,493 discloses a programmable flow controller that can be programmed with user-selectable flow characteristics, such as a function relating percentage flow rate to percentage valve stem position, which can be stored in firmware of the controller and can be used to calculate a correction factor for a control signal to a control valve positioner.
Although these techniques of controlling chokes may be useful, what is needed is a way to control a choke in a controlled pressure drilling system that better adapts to the flow characteristics and changes encountered in that environment.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.